1. A method for assaying surface-active peptides was developed. Phosphatidylcholine liposomes are prepared containing concentrated 6-carboxyfluorescein. When these liposomes are exposed to surface-active peptides, fluorescence is enhanced in proportion to the amount of peptide. The sensitivity is high; 50 picomoles of mellitin can be detected. The assay system was tested with fractions from HPLC runs on extracts of insect venoms. 2. Fluorescence polarization measurements of intrinsic protein emission have been made which show that rotational motion of the protein and of the individual tryptophan residues can be resolved. Calculations have been made to define conditions where intrinsic fluorescence is useful for measuring protein relaxation times.